Beam deflection tube with second



Aug. 7, 1951 H. NELSON BEAM DEFLECTION TUBE WITH SECONDARY EMISSIONELECTRODE Filed oct. so, 194e +1 awww @defi/v7' Patented Aug. 7, 1951BEAM DEFLECTION TUBE WITH SECOND- ARY EMISSION ELECTRODE Herbert Nelson,Bloomfield, N. J., assigner to Badio Corporation of America, a.corporation o! Delaware Application october an, 194s. sel-nil No.706,082

' 14 claims. (ci. 31a-ss) My invention relates to electron dischargedevices, and more particularly to an improved type of a beam deflectiontube.

One type of electron discharge tube utilizes a well denned electron beamwhich is moved across a narrow aperture of a shielding anode by a pairof deecting electrodes. To increase the transconductance of such a tubea dynode or secondary emission multiplier is used as an output anode.The dynode is placed beyond the aperture to intercept the electron beam.The output appears in the dynode to shield-anode circuit.

An object of my invention is to provide an electron discharge device ofthe beam deflection type having improved electrical characteristics.

More specically, it is an object of my invention to further increase thetransconductance of such tubes.

For this purpose, I have devised a novel arrangement of two secondaryemission multipliers which produce an alternating output current ofabout twice the amplitude for the same deflection voltage.

The novel features which I believe to be characteristic of my inventionare set forth with particularity in the appended claims, but theinvention itself will best be understood by reference to the followingdescription taken in connection with the accompanying drawing, in.lwhich:

Fig. 1 is a diagrammatic showing of an electron discharge tube accordingto my` invention;

Fig. 2 is a diagram showing the operation of a conventional beamdeiiection tube;

Fig. 3 is a diagram showing the operation of a beam deflection tubeaccording to my invention; and

Fig. 4 is a graphic representation of the operating characteristics of atube according'to my invention.

In Fig; l is shown a beam deilection tube which can be used as a mixertube in superheterodyn reception. The beam deflection tube forming thesubject matter of my invention is one whose basis of operation isdeection control of an electron beam across an apertured member followedby a secondary emitting target. This tube which embodies my inventionincludes an envelope I having at one end a cathode I2 providing aprimary electron beam. A cathode shield I4 surrounds the cathode on thesides and rear to aid in forml ing the beam. The shield I4 may beoperated at a bias or may be connected directly to the cathode. A beamfocusing and deiiecting electrode assembly I6 is positioned adjacent thecathode and biased at a suillciently high positive potential to producea stream of electrons from the cathode. Ihe structure. oi the assemblyI6 is designed in the form of a series of apertured barriers or grids.respectively numbered Il, I0 and 2| in Fig. 1, which are used to focusand center the beam. These grid-barriers have, respectively, aperturesIl, and 22, which are aligned to center the electron beam and acceleratethe beam toward a target assembly 30.

As is shown in Fig. l, barriers Il, I9 and 2| are constructed in asingle assembly in the tube A and are also operated at a common positivevoltage of preferably around 300 volts. Grids Il and I9 are designed toform and accelerate the elec-V tron beam. Between grid I8 and grid 2Iare placed two deiiectingl plates 24 and 26. These deflecting plates areinsulated from the assembly I6 and are used to deiiect the electron beamacross the opening 22 of grid 2l. For example, the local oscillator'voltage and the signal voltage 20 may be both applied across thedeection plates or if it is desired the local oscillator voltage may beapplied between the assembly I3 and one of the plates. while the voltagemay beapplied betweenl the grid-anode assembly I6 and the other plate.Furthermore, these deflecting plates 24 and 2B may be usedto direct theelectron beam at any one spot of the target assembly 30 by theapplication of an appropriate bias between the plates.

In the tube ot Fig. l. I have provided a novel l target assembly whichgives an increased output current. This target assembly includes anintercepting electrode comprising a plate 32 having an aperture 33 inalignment with the grid apertures Il, 20, and 22. Plate 32 is preferablymaintained at around 250 volts positive potential. The surface of plate32 facing the approaching electron beam is activated to have a secondaryemission ratio greater than unity. Thus, plate 32 is a secondaryelectron emitting electrode or dynode. A dynode in this case may bedefined as? an electrode having the property of emitting secondaryelectrons when struck by impinging primary electrons. A conductor lead36 connected to plate 32 is sealed through the glass envelope wall ofthe tube for connecting the electrode 32 in an output circuit. Thetarget assembly 30 further includes a target electrode in the form of acurved plate 3l placed directly behind the aperture 33 and preferablymaintained at 200 volts positive potential. Plate 3l in this positionwill receive the-impact of the electron beam whenever it passes throughthe aperture 33. The

. portion of plate 34 lntercepting the arriving electron beam is alsoactivated to have a ratio of secondary electron emission greater thanunity.

3 Thus, plate 34 comprises a second dynode or emitter electrode.

The first dynode or output electrode 32 is constructed with a beamintercepting portion extending across the beam` path and a portion 35forming a right angle with the electron beam intercepting portion togive the output electrode 32 an L-shaped cross-section. The seconddynode 34 preferably has a concave curvature facing the corner of theL-shaped dynode 32. This construction of the two dynodes provides anoverlying or covering of plate 34 by the output electrode 32.

The electron beam focused and accelerated by the electrode I6 will passthrough the aperture 33 and strike the curved surface of dynode 34. Thesecondary electron emission released from dynode 34 by this bombardmentof the electron beam will be drawn to the more positive output electrode32. In this manner the first dynode 32 provides a collector for thesecondary electron emission from the secondary electron emitter 34.

It is clear from Fig. -1 that the grid plate 2| of the assembly I6overlies the first dynode surface 32. Furthermore, since plate 2| ismaintained at a higher positive potential than the electrode 32, gridportion 2| will act as a collector of the secondary emission releasedfrom plate 32 by the bombardment of the impinging electron beam.

The operation of the tube of Fig. l is such that the electron emissionfrom cathode |2 will be formed into a beam and accelerated between thedefiector plates 24 and 26. The-fluctuation of the applied voltages toplates 24 and 26 will cause the electron beam to swing back and forthover aperture 22 onto the intercepting portions of plates 32 and 34. Asthe beam passes over the aperture 33 it will leave the interceptingsurface of the output electrode 32 and will fall upon the secondaryemitting surface of the second dynode 34. The aperture 33 islarger thanthe cross-sectional area of the electron-beam so that the beam inpassing through aperture 33 is not intercepted by any portion of plate32. When' the beam takes a reciprocating path back and forth overaperture 22 an alternating current will flow in the output circuit ofthe tube as'will-be shown below.

Fig. 4 graphically 'shows the variation of the input current of the.tube relative tothe changes inthe deilecting electrode voltages. '11,for example. a proper bias is established between plates 24 and 26,so'as to focus'the electron beam i'n a normal path entirely aperture 33on to dynode 34, there will be Va maximum flow of secondary electrons toelectrode 32 -'from the second dynode 34. This will establish 'acondition of maximum current ow'in one direction in the output circuit.This maximum current flow in the circuit of plate 32 may be representedby point B on the solid curve.

If signal or control voltages are singly or simultaneously appliedacro'plates 24 and 26, the electron beam will sweep back and forthacross aperture 33. The output current will be repre'- sented by thesolid curve onboth sides of point B. If the defiectionsare great'enugh,the first and second dynodes 32 and 34 will at a certain pointsimultaneously intercept the electron beam so that the electrons lost bythe secondary emission of plate 32 will equal those collected by plate32 from the primary beam and from the secondary emission of dynode 34.At thi's point the output current of the tube from plate 32 will becomezero. This condition of ze'ro output current' is represented by points Gand H o! the curve. A

ondary emitting surface of the output dynode' 32. At this point thesecondary emission leaving the electrode 32 will be at a maximum. Also,theoretically, there will be no electron flow to plate 32 except that ofthe impinging beam. The output current will then be a maximum in theother direction. This is represented by A, A' 'of the curve of Fig. 4. Astill greater deflection of the electron beam will cause the beam to becut oi entirely by the grid barrier 2| so that none of the beamelectrons will strike the target assembly 30. The output currentobviously drops to zero at this point which is represented by E,

E on the curve of Fig. 4.

Fig. 2 is a sketch of the target of a conventional beam tube asdisclosed in Fig. l2 of United States Patent No. 2,294,659 of E. W.Herold. A collector electrode 4|l corresponds to plate 2| of Fig. l. Theoutput electrode 42 is a positive plate corresponding to anode plate 32of Fig. l. The collector plate 43 has an aperture-44 for the passagetherethrough of an electron beam Ia.v The output plate 42 is positionedopposite aperture 44 and its surface upon which electron beam Inimpinges 'is' activated to have secondary emission greater than unity.Positioned in the center of aperture 44 is a post 46 of sumcient widthto intercept beam In as it is deected across aperture 44.

As the beam is deflected across the aperture 44. it strikes the outputelectrode 42 on both sides of the post 46. The current from electrode 42in the output circuit of the tube of Fig. 2 would be represented by thedotted curve and the upper portions of the solid curve of Fig. 4. Theoutput current would be zero as represented by point E of the curve,'where the electron beam IB falls entirely on collector plate 40. As thebeam is swung across the aperture 44, the current in the output circuitincreases to a maximum represented by point A of the curve and at whichthe electron beam Ia falls entirely upon the dynode 42. When theelectron beam is entirely intercepted by post 46, the plate currentfalls to zero as represented at O on the dotted curve. The currentrises, again to a maximum and thenfalls to zero as represented by pointsA and E', respectively, when the beam swings across the other half ofthe aperture 44 to the plate 40.

My tube of Fig. 1 provides an alternating output current as illustratedby the solid curve of Fig. 4, while the conventional type tube of Fig.2, provides an output current which flows only in one direction. Theamplitude of output current variation from the median value in myimproved tube is much greater than the corresponding amplitude obtainedin the conventional tube described. `In the conventional tube there isno provision touse the output electrode as a collector of secondaryemission from a second dynode.

The eiliciency of the tube of my invention will partially depend uponthe accuracy with which the electron beam is focused through aperture33.`

If the beam does not entirely pass through aperture 33 and if some ofthe beam simultaneously strikes the secondary emitting surface of plate32, the maximum current due to the collection of secondary electrons byplate 32 will be lessened.

Furthermore, the eiliciency of this tube will also depend upon howeffectively the output electrode 32 will collect the secondary emissionfrom dynode 34. If there is any escape of secondary electrons around theedges of plate 32 onto the accelerating anode assembly I6, the outputcurrent will be decreased by the amount of this electron loss. Thedesign of the collector plate 32 may conceivably be such as to preventany escape of secondary emission electrons from dynode 34 to parts ofthe tube other than plate 32. For example, plate 32 may enclose dynode34 on three sides or entirely on four sides. However, such arrangementswill greatly increase the capacitance between electrodes 32 and 34.

Figs. 2 and 3 indicate the output stages of respectively a conventionaltube and a tube incorporating my invention. In the conventional typebeam deflection tube, such as disclosed in the above mentioned Heroldpatent, the output stage is as shown in Fig. 2. The output electrode 42is maintained at a lower positive potential than that ofthe collectorileld plate 40. The primary electron beam IB is caused to impinge oneither 40 or 42, or simultaneously upon both. The net output current totarget electrode 42 is due to the diilerence between the primary beamelectrons reaching 42 and the secondary electrons leaving 42, and isequal to:

IT=RIB(dT-1) (1) where R is the fraction of the primary beam IB reaching42, and d'r is the secondary emission ratio of the surface of 42.Differentiation of (l) with respect to R gives the rate of change of Irwith R:

AII=IB 1T1 2) Thus, a finite deflection of the beam IB causing a changeAR in R causes a change in the output current to, or the net loss ofelectrons from, target 42 equal to IB(dT-1). Equations l and 2 applywhether the primary beam impinges in part on plate 40 or post 46.

In Fig. 3, field plate 2l, output electrode 32 and auxiliarysecondaryemitter 34 are maintained at progressively lower beam potentials. Theprimary electron beam IB is caused to impinge upon either 32 or 34 orsimultaneously upon both. The electron beam IB thus initiates secondaryelectron emission from dynode 34 which theoretically is collected bydynode 32. Also, the beam In initiates secondary electron emission fromelectrode plate 32 which in turn is collected by the field plate 2 I.Under the condition in which the beam IB falls on plate 32 or plate 34,or simultaneously upon plate 32 and 34, the net output current to outputplate 32 is due to the difference between the secondary electronsleaving 32 and the primary electrons and secondary electrons (from 34)reaching 32, and is equal to:

where R is the fraction of IB reaching 32, and dn, and ds are thesecondary emission ratios of 32 and 34, respectively. Differentiation of(3) with respect to R gives the rate of change of IT with R:

Equations 3 and 4 apply to Fig. 3 when the primary Ibeam is interceptedby `32 or 34, orboth. When the beam is intercepted by 2| or 34, ordivided between them, Equations 1 and 2, with di' replaced by dro,apply. Equation 4 shows 6. that when the beam is divided between 32 and34, a finite deflection of the beam causing a change AR in R causes achange in the output current to electrode 32 equal to Indro-Lfd.)

If the secondary electron emission ratios dr. dro and ds are eachassumed to be 4, a comparison of Equations 2 and 4 will show that theratio of Iro to Ir is 2%. This indicates that for the same change indeflection voltage in both tubes, there is theoretically more than twicethe current change or transconductance in the improved tube of Fig. 1.Due to several considerations, however, I have found that the ratio oftransconductance between the two tubes compared above is closer to 2.

A comparison of the transconductance of my improved tube with that ofthe conventional beam deflection tube is also illustrated in Fig. 4. Thetwo curves indicate that for a specic deflection voltage change'theoutput current of the improved tube will change in amount from B to Awhile in the conventional tube the same deflection voltage change willchange the output current only from O to A.

A further study of Fig. 1 and Fig. 4 will show that different resultsare produced by different vbiasing or focusing potentials placed uponthe deflecting plates 24 and 26. For example, if the electron beam isnormally focused entirely through the aperture 33 and if an alternatingdeflecting potential is imposed between plates 24 and 26, the outputcurrent will fluctuate about the point B as discussed above. However. ifthe electron beam is normally focused so as to fall entirely upon thefirst dynode 32 and if the beam is deflected by an alternating voltagebetween plates 24 and 26, the output current will vary on either sidesof points A or A', of the solid curve of Fig. 4. Under either of thesetwo conditions with the electron beam focused entirely upon plate 32 orupon plate 34, the output current with no deflection will be a maximumin one direction as shown respectively by points A, A', and B. At thesetimes, it is evident that the transconductance of the tube is zero.

However, it is possible that the beam be focused in a normal path suchthat part of the beam falls on dynode 32 and the rest of the beam fallson the second dynode 34. In this case the effective plate current willbe zero and can be represented by points G or H of Fig. 4 as discussedabove. Fig. 4 indicates that under this condition the transconductanceof the tube will be at a maximum so that for small deflecting voltagesapplied between plates 24 and 26 there will be large changes in theplate output current.

While certain specific embodiments have been illustrated and described,it will be understood that various changes and modifications may be madetherein without departing from the spirit and scope of the invention.

What I claim as new is:

1. An electron discharge device comprising means for producing anddirecting a beam of electrons along a normal path, means for deilectingsaid electron beam away from its normal path, a first and a seconddynode spaced along the path of the beam in the order named, each ofsaid dynodes having a portion for intercepting said electron beam, eachof said portions having a ratio of secondary electron emission greaterthan unity. said first dynode having a portion arranged to intercept thesecondary emission initiated by the electron beam from said second dy- 7node, means for collecting the secondary emission initiated by theelectron beam from said nrst dynode, and means for connecting said ilrstdynode in an output circuit, whereby a varying output current will beproduced in the output circuit.

2. Anr electron discharge device comprising means for producing anddirecting a beam oi.' electrons along a normal path, an output electrodeand an emitter electrode, said electrodes spaced along the path of saidelectron beam in the order named, each of said electrodes having aportion for intercepting said electron beam, said intercepting portionsoi' said electrodes being activated to have a secondary electronemission ratio greater than unity, said output electrode forming acollector for the secondary emission from said emitter electrode, meansfor collecting the secondary emission from said output electrode, andmeans for deilecting said beam across the intercepting portions of saidelectrodes. v

3. An electron discharge device comprising means for producing anddirecting a beam of velectrons along a path, means for dedecting saidelectron beam away from said directed path, an emitter' electrode havinga portion intercepting said electron beam at one portion of said beam,an output electrode having a portion intercepting said electron beam atanother portion of said beam, said beam intercepting portions of saidelectrodes being activated to have a secondary electron emission ratiogreater than unity, said output electrode having a second portionpositioned to collect the secondary emission from said emitterelectrode, and means for collecting the secondary emission from saidoutput electrode.

4. An electron discharge device comprising means including an electronsource for producing and normally directing a beam of primary electronsalong a beam path, a target assembly for said beam comprising an outputelectrode adapted to be part of an output circuit and an emitterelectrode, said electrodes being spaced along the normal path of saidelectron beam, said output electrode comprising an aperturedintercepting portion positioned between said emitter electrode and saidelectron source for permitting the undeilected electron beam to passthrough the aperture thereof to said emittingr electrode and forintercepting at least a portion of said electron beam when the latter isdeflected from said normal path, deflection means for moving said beamacross the aperture of said intercepting portion of said outputelectrode, said emitter electrode having an intercepting portionpositioned back of said aperture to intercept the portion of said beampsing therethrough, said beam intercepting portions of said electrodesbeing activated on the side facing the arriving electron beam to have aratio of secondary electron emission greater than unity, collector meansfor intercepting the secondary emission from said output electrode, saidoutput electrode forming a collector for the secondary emission fromsaid emit ter electrode whereby a varying output current will beproduced' in said output circuit as the electron beam is deflectedacross the aperture of said output electrode.

5. An electron discharge device comprising means including an electronsource for,producing and normally directing a beam of primary electronsalong a path, a target assembly for said beam comprising an outputelectrode adapted to be connected in an output circuit and an emitterelectrode, said electrodes spaced along the normal path of said electronbeam. a portion of said output electrode positioned between said emitterelectrode and said electron source for intercepting said beam. means fordeecting said beam away from its normal path over saidinterceptingportion of said output electrode, said intercepting portion having anaperture positioned in the normal path of said deilected beam, focusingmeans for directing said electron beam along its normal path throughsaid aperture, said emitter electrode being positoned to intercept saidelectron beam when directed through said aperture, said electrodes beingactivated on the beam intercepting portions thereof to have a ratio ofsecondary electron emission greater than unity, said output electrodeoverlaying the emitting portion of said emitter electrode for collectingthe secondary emission thereof. said beam focusing means including aplate element adjacent the emitting portion of said output electrode forcollecting the secondary emission of said output electrode, whereby avarying output current will be produced in the output circuit as theelectron beam is deilected from its normal path across the aperture ofsaid output electrode.

6..An electron discharge device comprising means including an electronsource for producing and normally directing a beam of primary electronsalong a path. a target assembly for said beam comprising an outputelectrode adapted to be connected in an output circuit and an emitterelectrode, said electrodes spaced along the normal path of said electronbeam. a portion of said output electrodelpositioned between said emitterelectrode and said electron source for intercepting said beam, means fordeecting said beam away from its normal path over said interceptingportion of said output electrode, said intercepting portion having anaperture positioned in the normal path of said electron beam, focusingmeans for directing said electron means along its normal path throughsaid aperture, said emitter electrode being positioned to intercept saidelectron beam when directed through said aperture, said electrodes eachhaving the surface of the beam intercepting portions thereof activatedto have a ratio of secondary emission greater than unity, said outputelectrode having a second portion extending across the activated sur--face of said emitter electrode for collecting the secondary emissionthereof, said beam focusing means including a plate element adjacent theemitting surface of said output electrode for collecting the secondaryemission of said output electrode, whereby a varying output current willbe produced in the output circuit as the electron beam is deflected fromits normal path across the aperture of said output electrode.

7. An electron discharge device comprising means including an electronsource for producing and directing a beam of primary electrons along apath, a target assembly for said beam comprising an output electrodeadapted to be connected in an output circuit and an'emitter electrode,said electrodes spaced along the path of said electron beam, a portionof said output electrode positioned between said emitter electrode andsaid electron source for intercepting said beam, deflecting means formoving the beam over said intercepting portion, focusing means fordirecting said electron beam to any point of the mtercepting portion ofsaid output electrode, said intercepting portion having an aperturetherein, said emitter electrode being positioned to intercept saidelectron beam when said beam trode overlaying the emitting portion ofsaidl emitting electrodes for collecting the secondary emission thereof,said beam focusing means having an element .thereof adjacent theemitting portion of said output electrode for collecting the secondaryemission of said output electrodes, whereby an alternating outputcurrent will be produced in the output circuit as the electron beam isdeiiected across the aperture ot said output electrode.

8. An electron discharge device comprising means including an electronsource for producing a beam of primary electrons, an output electrodeincluding a portion extending transversely of the path of said beam andarranged for intercepting said electron beam, said portion having anaperture, means for focusing said electron beam through said aperture,an emitter electrode spaced behind said output electrode relative to theelectron beam direction, said emitter electrode having a portion forintercepting said electron beam passing through said aperture, each ofsaid beam intercepting portions of said electrodes being activated tohave a ratio oi' secondary emission greater than unity, deiiecting meansfor moving said electron beam across the` aperture between activatedportions of said output electrode, said beam focusing means including apart for collecting the secondary emission of saidoutput electrode, saidoutput electrode forming a collector for the secondary emission fromsaid emitter electrode.

9. An electron discharge device comprising means including an electronsource for-produc ing a. beam of primary electrons, a target assemblyfor said beam including an output electrode and an emitter electrode,said electrodes extending transversely of the path of said beam, saidoutput electrode spaced in front of said emitter electrode relative tosaid electron beam 'direction and in a position that said outputelectrode intercepts a portion of said beam and said emitter electrodeintercepts the remainder of said beam, deflection means for moving saidbeam over said target electrodes whereby the amount of current impactingon one electrode is decreased and the amount oi beam current impactingthe other electrode is simultaneously increased to an equal amount, eachof said electrodes being activated on the side facing the arrivingelectron beam to have a ratio of secondary electron emission greaterthan unity, said output electrode having a portion for collecting thesecondary emission from said emitter electrode when said outputelectrode has a higher positive potential than said emitter electrode,and said beam producing means including a portion for collecting thesecondary emission from said output electrode when said beam producingmeans is at a higher positive potential than said output electrode.

10. An electron discharge device comprising means including an electronsource for producing a beam of primary electrons, an output electrodeincluding a portion extending transversely of the path of said beam tointercept said electron beam, said electrode portion having an aperture,means for focusing said electron beam through said aperture, an emitterelectrode spaced behind said output electrode relative to the electronbeam direction, said emitter electrode having a portion for interceptingsaid electron balm passing through said aperture, each of said beamintercepting portionsoi" said electrodes being acing a beam of primaryelectrons, an output electrode including a portion extendingtransversely of the path of said beam to intercept said electron beam,said electrode portion having an aperture, means for focusing saidelectron beam through said aperture, an emitter electrode spaced behindsaid output "electrode relative to the electron beam direction, saidemitter electrode having a portion for intercepting said electron beampassing through said aperture, each of said beam intercepting portionsof said electrodes being activated to have a ratio of secondary emissiongreater than unity, deiiecting means for moving said electron beamacross the aperture between activated portions of said outputelectrodes, means for collecting the secondary emission of said outputelectrode, said output electrode forming a collector for the secondaryemission from said emitter electrode.

12. An electron discharge device comprising means for producing anddirecting a beam of electrons along a normal path, means for deilectlingsaid electron beam away from its normally directed path, a targetelectrode spaced from said beam producing means and extending acrosssaid normal beam path, an intercepting electrode having a portion spacedfrom the normal path of said electron beam for intercepting a portion oisaid electron beam when deflected from its normal path, a collectorelectrode having a portion spaced farther from the normal path of theelectron beam than said intercepting electrode portion being, saidcollector electrode portion mounted transverse to the beam path forintercepting a portion of the electron beam when the beam is deflectedbeyond said intercepting electrode portion, said target and interceptingelectrodes being activated to have a secondary electron emission ratiogreater than unity.

13. An electron discharge device comprising means for producing anddirecting a beam of electrons along a normal path, means for deflectingsaid electron beam away from its normally directed path, a targetelectrode spaced from said beam producing means and intercepting saidnormal beam path, an intercepting electrode having a portion with anaperture coaxial with the normal electron path, said portionintercepting a portion of said electron beam when deflected from itsnormal path, a collector spaced farther from the normal path of saidintercepting electrode portion for intercepting a portion of theelectron beam when the beam is deflected beyond said interceptingelectrode portion, said target and intercepting electrodes beingactivated to have a secondary electron emission ratio greater thanunity.

14. An electron discharge device comprising means including an electronsource for producing and normally directing a beam of primary electronsalong a beam path, a collector electrode spaced along the beam path andhaving an aperture encircling the beam path, a iirst dynode electrodehaving a portion interceptingV the beam path through said collectoraperture from said electron source, said dynode portion having anaperture encircling the beam path, a second dy- Il l 12 ii'hnode saidbeand path REFERENCES CITED roug said eeperture fromsa elecon source. umt um md mod being m''hgf fllsownanreierences are of record in theactivated to have a. seeondxry emission ratio greater than unity, meansfor detiectins said 5 UNITED STATES PATENTS electron bem away from thebeam path onto Number Name Date nid collector electrode, means forconnecting said 2,164,892 Banks v July 4, 1939 nrst dynode into anoutput circuit whereby a 2,210,034 Keystnn Aug. 6. 1940 varying outputcurrent will be produced in the 2,257,795 Gray Oct. 7, 1941 outputcircuit. l0 $942,659 Herold Sept. 1, 1942 mm1' NELSON. 2,393,803 NelsonJa'n. 29, .1946

